1. Field of the Invention
This invention relates to laser amplifiers employing phase locked phase conjugation, and to a system for providing such phase locked phase conjugation.
2. Discussion of Prior Art
Beams of laser light may be amplified by passing a beam through an amplifying medium which is pumped with light energy, eg a flash lamp. The beam interacts with pump energy in the amplifying medium and an amplified output is obtained. It is known to reflect an amplified beam by a mirror so that the laser beam passes twice through the amplifier with consequential gain. There are several limits upon the amount of amplification that can be obtained. Increasing the cross section area and or length of the medium causes problems of cost and medium inhomogeneity. Another problem concerns heat build up in the medium.
To overcome the above, it is known to split a laser beam into several beams and pass each sub-beam through amplifiers. The amplified sub-beams are recombined. Unfortunately, phase coherence is lost in this arrangement.
Several attempts have been made to split, amplify, and recombine a laser. Some attempts have used phase locked phase conjugation techniques (phase locked phase conjugation has also been referred to as "beam combining" or "coherent coupling").
Phase locked phase conjugation is a variant of the technique of optical phase conjugation. In optical phase conjugation a "time reversed" replica of an optical beam is produced when the beam is incident upon a "phase conjugate mirror" (PCM). The time reversed, or phase conjugate, beam exactly retraces the path of the original beam.
One application of phase conjugation is for removing abberations in a double pass of a laser amplifier (D. A. Rockwell. "A review of phase-conjugate solid-state lasers", IEEE J. of Quantum Electron., 24, p1124 (1988)).
In another arrangement a laser beam was divided by a beam splitter mirror into two sub beams and each sub beam directed through an amplifier into a simulated Brillouin scattering (SBS) mirror. The SBS mirror reflects the two beams back through the amplifiers and onto the beam splitter mirror. Unfortunately the two amplified beams will have randomly different phases. As a result, when the amplified beams return to the beam splitter only part of the beams will go back along the direction of the original beam. Instead randomly different amounts will reflect off the beam splitter and pass through it. Use of independent PCMs will not give phase locking with amplification. Phase locking with phase conjugation has been reported by Basov (N. G. Basov, V. F. Efimkov, I. G. Zubarev, A. V. Kotov, S. I Mikhailov, and M. G. Smirnov, "Inversion of wavefront in SMBS of a depolarised pump", JETP Lett. 28, p197 (1978)). In these experiments a single depolarised beam was split into two beams, the two beams were directed on to an aberrator, which was imaged into an SBS cell containing a waveguide. The function of the abberator and the waveguide is to obtain a high degree of overlap between the two beams so that they are reflected from a common SBS grating within the SBS medium.
Basov et al also described phase locking with up to nine beams. This was achieved by using a waveguide to get overlap between the beams. An auxiliary reference beam was also used to improve phase locking. Problems associated with these schemes involving waveguides are less than perfected phase conjugate fidelity (near field speckle on the output beams is a particular problem), and low reflectivity (the maximum reflectivity achieved in Basov was 15%). Another limitation with using a waveguide is that the refractive index of the SBS medium must be greater than that of the waveguide material in order for waveguiding to take place. This places severe restrictions on the choice of SBS media. Another known system is described in D. A. Rockwell and C. R. Giuliano, "Coherent coupling of laser gain media using phase conjugation", Optics; Letters, 11, p147 (1986) Carroll et al, Journal of the Optical society of America B, Vol. 9, p.2214, (1992. It does not use a waveguide; and/or abberator, but achieves overlap by focusing the beams down with a lens and making them parallel so that they overlap in the focal region of the lens (the far field) and thus share a common SBS grating. The major disadvantage of this technique is a very high degree of sensitivity to misalignment (S. Sternklar, D. Chomsky, S. Jackel, and A. Zigler, Misalignment sensitivity of beam combining by stimulated Brillouin scattering.", Optics Letters, 15, p.469 (1990)), which makes it difficult to use in a practical system".
Another known system has been published by N. F. Andreev, E. A. Khazanov, S. V. Kuznetsov, G. A. Pasmanik, E. I. Shklovsky and V. S. Sidorin, IEEE J. of Quantum Electron., Vol 27, p135 (1991). This is based on four wave mixing between two beams that have orthogonal polarisation states. This technique gives good phase locking and phase conjugate fidelity, as well as being relatively insensitive to misalignment. Its main drawback is that it requires increased complexity if more than two beams are to be phase locked.